1. Field of the Invention
The invention relates to a method for locating the interface between two fluids in an underground cavity.
2. Description of the Prior Art
In a typical well arrangement penetrating a salt cavity, there is, for example, protective casing cemented to the wall of the cavity with a second smaller diameter casing string concentric with the protective casing but penetrating deeper, and a third, even smaller casing string, concentric with the second casing string and penetrating yet further into the cavity.
In the above arrangement, the second casing string and the wall of the cavity form an annulus below the shoe of the cemented protective casing. In "washing out" such a salt cavity to enlarge it for use as a storage cavity, oil is placed into the annulus formed by the second casing string and the wall of the cavity. The bottom level of the oil is some distance above the shoe of the second casing string. Water is then pumped down the third (innermost) casing string into the cavity to dissolve salt from the cavity. The salt saturated water (brine) is then removed through the annulus formed between the second and third casing strings.
A known method of determining the interface between oil and brine in the annulus defined between the wall of the cavity and the second casing string is by gamma ray measurements, wherein the oil or the brine or an intermediate layer disposed in the interface area is provided with a source of gamma rays. The density of this intermediate layer which is to drop into the annulus through the one medium, preferably the oil, to the level of the other medium, preferably the brine, must range between the densities of the first and the second media and must not dissolve too quickly in either of these media. Thus, according to one known method synthetic balls containing a source of gamma rays are employed as the intermediate layer between the oil and the brine. Another known method employs labeled fresh water.
As mentioned before, when cavities are washed out, the annulus defined by the wall of the cavity and the second casing string contains an oil preventing a washing-out of the upper area of the salt formation. In order to avoid the production of oil with the brine through the annulus between the second and third casing strings, the interface between the oil and the brine is maintained if possible some distance above the shoe of the second casing string. The use of plastic balls comprising a source of gamma rays involves the disadvantage and risk that as the oil level drops to the level of the shoe of the second casing string, the plastic balls become stuck in the annulus between the second and third casing strings, and as a result the plastic balls will not follow the movements of the interface. Moreover, it is possible that with the dropping of the interface, the plastic balls pass into the interior of the protective tube string and if not stuck are produced with the oil-containing brine in an uncontrolled manner to the surface.
Likewise, the use of labeled fresh water involves uncertainties. If labeled fresh water is pumped into the annulus between the wall of the cavity and the second casing string, it divides itself into drops of various sizes. As a result, the drops fall through the oil to the surface of the brine at greatly varying rates. This labeled fresh water arriving at the upper level of the brine dissolves salt from the rock, thus becoming heavier and diffusing into the salt water filling the cavity. Therefore, at the time of measurement there may be insufficient labeled water available at the interface to determine its level.
It is an object of this invention to provide a method for determining by gamma-ray measurements an interface between media, especially between oil and brine, during the washing-out and operation of underground cavities, said method avoiding the afore-mentioned disadvantages of the prior art methods.